1. Field of the Invention
The present invention relates to an image forming apparatus such as a copy machine, a printer, and a multifunctional peripheral. More particularly, the present invention relates to an image forming apparatus for forming a toner image by providing a latent image formed on an image holding body with reversal development using a two-component developer which is made of toner and carrier, and a control method thereof.
2. Description of the Related Art
An image forming apparatus according to a related art includes a photoreceptor as an image holding body, a charging device for charging the surface of the photoreceptor to have a predetermined voltage, an exposure device for forming an electrostatic latent image by exposing the surface of the photoreceptor with a light source such as a laser element and a LED, and a developing device for developing the electrostatic latent image. The electrostatic latent image formed on the surface of the photoreceptor is developed by the developing device to be a toner image. The developing device develops the photoreceptor using a two-component developer containing toner which has undergone frictional charging and carrier.
According to the related art, it is required to prevent toner and carrier that unnecessarily become airborne during development from adhering to the photoreceptor in order to prevent degradation of an image. Therefore, in the image forming apparatus according to embodiment of the present invention, setup of operations timing such as charging the photoreceptor, application of a developing bias and driving the developing roller are important. Therefore, there have been proposed various setting methods.
FIG. 1 illustrates an ordinary method of setting an action of the image forming apparatus according to the related art. In the example of FIG. 1, a developing bias voltage is started to be applied to a developing roller at a time when a charged region of a photoreceptor, which is charged by a charging device, reaches a position facing the developing roller. Further, the developing roller is started to be moved at a time when an electrostatic latent image (image unit) on a surface of the photoreceptor, which is exposed to light in correspondence with image information by an exposure device, reaches a position facing the developing roller.
Reference sign S1 in FIG. 1 designates a turnaround time until the charged region of the photoreceptor reaches the position facing the developing roller. Reference sign S2 designates a turnaround time until the electrostatic latent image formed on the surface of the photoreceptor reaches a position facing the developing roller.
According to the setting method of the related art, the developing roller is driven only when the electrostatic latent image on the photoreceptor reaches a position facing the developing roller. Therefore, it is possible to minimize events such that toner and carrier on the developing roller become unnecessarily airborne and moves toward or adheres to the photoreceptor with an electrical potential difference between the developing roller and the photoreceptor.
FIG. 2 schematically illustrates the electric potential of the surface of the photoreceptor when the rotating photoreceptor passes the position facing the developing roller. FIG. 2 illustrates a case where the photoreceptor and the developing roller are operated at a timing illustrated in FIG. 1.
Referring to FIG. 2, the photoreceptor is uniformly charged to have an electric potential VH by the charging device, and a developing bias voltage VB is applied to the developing roller. At a timing t1 of FIG. 2, an uncharged region (0 V) of the photoreceptor, which is not charged by the charging device, passes through the position facing the developing roller. At timing t2 of FIG. 2, a charged region of the photoreceptor reaches the position facing the developing roller. At timing t3 of FIG. 2, the charged region of the photoreceptor passes through the position facing the developing roller. At timing t4 of FIG. 2, the developing roller is started to be driven. Reference sign K designates a carrier adhered onto the photoreceptor.
Further, after the developing roller is started to be driven, the electric potential of a portion of the latent image is depressed with exposure of light for forming an image by the exposure device. The electric potential of the depressed portion of the latent image becomes a voltage VL of the latent image. The depressed portion reaches the position facing the developing roller. Toner T contained in the two-component developer is electrostatically attracted onto the latent image by an action of the developing bias voltage.
When the various actions are done at the above-mentioned timings t1 thru t4 and so on, the developing roller may be stopped while the photoreceptor charged by the charging device rotates. The developing roller stops between the timing t2 and the timing t4.
FIGS. 3A and 3B illustrate operations of the photoreceptor in the image forming apparatus of the related art. As illustrated in FIG. 3A, when the photoreceptor 30 rotates along with developer D adhered onto the photoreceptor 30 in a development area 31 between the photoreceptor 30 and the developing roller 32, the developer D may be pulled toward a downstream side of a rotational direction of the photoreceptor 30.
Referring to FIG. 3B, when the developing roller 32 starts to be driven while the developer D is pulled toward the downstream side of the rotational direction, the developer D held by an action of the magnetic force of the developing roller 32 is pulled outside a range where the action of the magnetic force is effective. As a result, the action of magnetic force of the developing roller 32 does not reach the developer D. Then, the developer D is separated from the developing roller 32 and electrostatically attracted by the charged region of the photoreceptor 30. The attracted developer D is shaped like a belt along an axial direction of the photoreceptor 30, and adheres to the photoreceptor 30.
As examples of the related art, Patent Document 1 and Patent Document 2 disclose that carrier is prevented from adhering to a photoreceptor by driving a developing roller before a charged region of a photoreceptor reaches a position facing the developing roller. Further, Patent Document 3 discloses a developing device equipped with plural developing rollers.
Patent Documents 4 and 5 disclose that the angle of a developing magnetic pole unit of a developing device having two developing rollers, a gap between the developing rollers and a photoreceptor, and a doctor gap between a doctor blade and the developing rollers are determined.
However, there is a likelihood that the carrier adheres to a region other than the position facing the developing roller on the surface of the photoreceptor. Further, the carrier airborne inside the developing device may intrude into a cleaner to thereby damage or degrade the photoreceptor during the cleaning.
When the developer is degraded and the doctor blade wears, the developer may clog a development gap provided between the developing rollers and the photoreceptor, and a phenomenon may occur that the photoreceptor is stopped rotating. Hereinafter, such stopping of the photoreceptor is referred to as photoreceptor rotation locking. Further, the photoreceptor rotation locking is apt to occur when the image forming apparatus repeats starting and stopping operations at short cycles by continuously repeating intermittent printing.
There is a so-called fountain type developing device including a developing roller which is rotatable in a direction the same as that of a photoreceptor and is positioned on an upstream side, and another developing roller which is rotatable in a direction reverse to that of a photoreceptor and is positioned on a downstream side. The photoreceptor rotation locking is especially apt to occur at a position between the developing roller which is rotatable in the direction the same as that of the photoreceptor and the photoreceptor. Further, the photoreceptor rotation locking is especially apt to occur when a processing speed of forming an image is high. Therefore, it is necessary to consider appropriate values of a development gap, a magnetic pole angle or the like in addition to drive controls of the development gap and the magnetic pole angle.
However, these problems of adhesion of the carrier, image degradation, and photoreceptor rotation locking are not considered to be solved in Patent Documents 1 and 2. Further, it is not at all disclosed in Patent Documents 4 and 5 that timings of starting to rotate the photoreceptor and the developing roller are controlled. Patent Document 3 does not disclose that a timing of starting to rotate the photoreceptor and a timing of starting to rotate the developing roller are controlled, nor how the magnetic pole angle or the like is set.
The developing magnetic pole unit having a structure illustrated in FIG. 9 is partly described in Patent Document 6. However, controls of timing of starting rotation of a photoreceptor and timing of starting to rotate a developing roller are not disclosed in Patent Document 6.    Patent Document 1: Japanese Patent No. 3624666    Patent Document 2: Japanese Patent No. 3203677    Patent Document 3: Japanese Unexamined Patent Application Publication No. 2008-257225    Patent Document 4: Japanese Unexamined Patent Application Publication No. 2008-129415    Patent Document 5: Japanese Unexamined Patent Application Publication No. Hei. 11-258905    Patent Document 6: Japanese Unexamined Patent Application Publication No. Hei. 8-36300.